As integrated circuits become more complex and packing density increases, difficulty of interconnecting the individual circuits, circuit elements and groups of circuits together on a substrate, also becomes more complex. At present, multiple layers of interconnecting metal strips separated by an insulator are utilized, which strips are connected to the underlying circuit elements through contacts. This is referred to as a multilevel interconnect system. Typically the multilevel interconnects are fabricated on a semiconductor substrate from polycrystalline silicon, aluminum or some type of refractory material. The substrate can be any type of semiconductor material such as silicon, gallium arsenide or any III-V compound or Mercury Cadmium Telluride or any II-VI compound.
In fabricating multilevel interconnects, it is necessary to sequentially form each layer of interconnect by either depositing or sputtering a layer of conductor onto the substrate followed by a patterning step and then overlying that layer with a layer of insulator followed by the next layer of conductors. This multilevel interconnect system presents some disadvantages as the integrated circuit becomes more dense, since the surface topology tends to become more irregular. This requires the interconnect levels to follow the topology of the surface. An additional disadvantage in the multilevel interconnect system is that each layer has its own adhesion problems that can result in the delamination of layers from one another. Another disadvantage to the multilevel interconnect system is that contacts must be brought up from the surface where the circuit elements are formed in order to make the interconnection.
Another technique to alleviate the problem of multilevel interconnects is to provide buried regions of conductive material which are sometimes referred to as "buried tank" of semiconductor material. These buried tanks are utilized for short conductive runs, since the resistivity of doped semiconductor material is relatively high compared to the conductor strips utilized in the multilevel interconnect techniques.